Well, seeing as both play a role, you should find ways to seperate them.
I'd say you need 3 setups, the default one, yours, and a setup giving fast
times to other(s). Find a fast guy (ideally already a friend..) and trade
setup with him. See if:
1-you are faster than usual with his setup
and/or,
2-he is slower than usual with your setup
If any of these are true, i'd say you can gain by tuning your setup.
If on the other hand there is no clear difference, then maybe your
driving skills are the limiting factor. Driving too carefully for example.
I find messing with the FOV can do wonders with your perception
of speed as you approach turns. I always set my FOV as high as
possible (some people can't stand too high) so things 'appears faster
than they are'. It's helped me take turns faster by reducing the time
i lost understeering at the entry.
Sometimes being faster is all about confidence in a predictable setup.
Some 'fast' setups are considered undriveable by the majority while
some 'slower' ones might be much more confortable and allow you
to be much more consistent. It's like that tale about the Turtle and
the Rabbit, it's not only about being fast, it's about finishing ahead
Put another way, it's not the laptime that's important to win, but
the overall time. This is where consistency is very important. I got many
top3 positions by simply hanging out through the entire race. Preserving
my tires, trying to keep acceptable (if not the fastest) times and repeat
those times over and over. I usually concentrate more on my braking
points and look where i need to gain speed, trying different ways to take
a turn (usually playing around with the apex) and noting the lap times.
Next thing you know the race is over and you are 2nd or something while
the fast guys are arguing who crashed who.
In any case, it's not a simple thing to pinpoint, but at the same time, it
is one of the major factor in the staying power of LFS. The fact that you
can't find the perfect setup in a week just makes it more fun in a way.